A variety of systems for detecting such a resonant circuit have previously been disclosed and utilized commercially with varying degrees of success. For example, a pulsed system such as described above is disclosed by Thompson (U.S. Pat. No. 3,740,742). The primary advantage of such a system is that it is much easier to detect the relatively weak signals generated by the marker circuit in the absence of much stronger fields produced by the transmitter. Other techniques for detecting the weaker marker signals over the much more intense transmitted signals include the detection of signals at frequencies other than that originally transmitted, such as by use of a marker which generates harmonics of the transmitted frequency. Similarly, it is known to sweep the transmitted energy over a range of frequencies encompassing the resonant frequency of the marker circuit such that the marker may be detected by conventional grid-dip techniques. As depicted by Burpee et al. (U.S. Pat. No. 3,810,172), it is also known to transmit a plurality of discrete frequencies, such as five, to allow for variation in the actual resonant frequency of targets or for change of resonance which might occur due to the presence of metallic bodies or other loading. In an extension of such a multi-frequency technique, Wahlstrom (U.S. Pat. No. 4,023,167) depicts a system in which each tag carries a number of circuits, each resonant at a different frequency, thus enabling each tag to be individually identified. That disclosure further suggests that the receiver may be tuned along with the transmitter and that a background signal may be detected when no tag signal is present, stored, and substracted from tag signals.